The present invention relates generally to rechargeable batteries and to electronic circuits for charging rechargeable batteries.
Rechargeable batteries have become quite popular for use in electronic equipment, power tools, portable computers, cordless phones, children's toys, and the like. Virtually every user of rechargeable batteries wants the ability to recharge as quickly as possible. In addition, some users like the convenience of being able to leave a rechargeable battery in the charger at all times, so that a fully charged battery will always be available. To accommodate the former requirements, there are a number of quick-charging battery chargers available. To accommodate the latter, there are a number of trickle charging battery chargers available.
Whether the battery charger is designed as a quick-charger or as a trickle charger, it is highly desirable to avoid overcharging. Overcharging produces undesired heating and high pressures which can physically and chemically alter the cells of the battery and degrade the battery's capacity to hold charge. This problem is particularly prevalent when quick-charging battery chargers are used. Quick-charging battery chargers typically deliver a high charging current which can rapidly overheat the battery unless the charging current is terminated or greatly reduced once the fully charged condition is reached.
Determining precisely when to terminate the high charging current is not simply a matter of sensing when the battery voltage reaches a fully charged level. In most batteries the voltage rises in a nonlinear way as charging current is applied, and it is often difficult to accurately sense or predict when the fully charged voltage is reached.
The Applicant's assignee has devoted a considerable effort in analyzing the voltage characteristics of rechargeable batteries as charging current is applied. It is now known that the battery voltage increases over time as charging current is applied and that the voltage-time curve exhibits various inflection points where the slope of the first derivative curve of the voltage variation with time actually changes from positive to negative or from negative to positive. The Saar et al. U.S. Pat. Nos. 4,388,582 and 4,392,101, assigned to the assignee of the present invention, describe these inflection points in conjunction with a rapid charging system for rechargeable batteries.
Although the inflection point analysis technique described in the Saar et al. patents has been widely successful, there is still interest in further improvement. Today the Saar et al. technique is frequently implemented using microprocessor or microcontroller circuitry which periodically samples the battery voltage during charging and uses that sample data to perform an inflection point analysis. Analog-to-digital converters are used in sampling the battery voltage data and converting that data into digital values which the microprocessor manipulates. The Assignee's current technology uses logarithmic analog to digital converters for this purpose.
Inasmuch as the sampled voltage data is already being obtained for use by the Saar inflection point analysis technique, that data can also be used to perform other charge termination techniques. One such technique is the negative slope technique which seeks to terminate charging current when the slope of the voltage-time curve becomes negative. The falling slope technique can be used in place of the Saar inflection point technique or it can be used to augment the Saar technique. One problem with the falling voltage or negative slope technique is that it can be quite sensitive to noise. Noise is particularly troublesome in the flat regions of the voltage-time curve, since a momentary drop in voltage due to noise can be misinterpreted as a falling voltage or negative slope.
The present invention solves the noise problem by automatically changing the effective sampling rate during the flat region of the voltage-time curve to obtain higher noise immunity. Modulation of the sampling rate to improve the noise immunity may be effected by sensing the slope of the voltage-time curve and automatically decreasing the sampling rate when the shallow portion of the curve is reached. In accordance with one aspect of the invention, the sampling rate can be effectively decreased by switching off the falling voltage slope termination technique during a predetermined charging interval corresponding to the flat region of the curve. Turning off the charge termination routine in this fashion has the effect of increasing the sampling interval to a time longer than the flat portion of the curve. Thus any noise-induced falling voltage fluctuations during this interval are ignored.
In accordance with another aspect of the invention, the sampling rate during the flat portion of the curve can be decreased by suitable manipulation of the voltage readings stored on the microprocessor memory stack. Using this technique, samples of the battery voltage are continually taken at predetermined intervals and stored on the stack. A slower sampling rate is effected during the flat portion of the curve by using every other stored value, every third stored value, every fourth stored value, and so forth, to achieve the desired sampling rate modulation. If desired, adjacent stack values can be averaged thereby further mitigating the effects of noise.
Accordingly, in its simplest form, the present invention solves the aforementioned problem associated with the falling voltage slope termination technique by automatically disabling that technique when the signal to noise ratio is poor (i.e. when the voltage-time curve is comparatively flat). At other times, the falling voltage slope technique is enabled, allowing the battery charging to be terminated when the battery achieves full charge and the voltage begins to drop. In this way, the falling voltage slope technique can be used in conjunction with the Saar inflection point technique or as a replacement for it.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.